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Fluids and Flow
Published in Sarah Armstrong, Barry Clifton, Lionel Davis, Primary FRCA in a Box, 2019
Sarah Armstrong, Barry Clifton, Lionel Davis
A pneumotachograph is a constant-orifice, variable-pressure flow meter. It is used in anaesthetic circuits therefore must cause little low resistance to gas flow. It contains a pneumotachograph head, of which there are several types The screen pneumotachograph has a gauze screen that the gas flows through; this produces a pressure drop across the screen. Flow must be laminar for it to function correctly. Pressure ports on either side of the screen deliver the pressure drop to a transducerThe Fleisch pneumotachograph uses a series of fine-bore ducts to guarantee laminar flowThe hot-wire pneumotachograph has two heated wires perpendicular to each other across the lumen of the head. The gas flow cools the wires, which alters their resistance and produces an electrical signalThe modified Pitot tube pneumotachograph contains two small-pressure, open-ended sampling tubes positioned in the centre of the gas flow channel. The pressure difference between the upstream (dynamic) and downstream (static) ports depends on the kinetic energy of the gas, which is proportional to the (velocity)2
Equipment and monitoring
Published in Brian J Pollard, Gareth Kitchen, Handbook of Clinical Anaesthesia, 2017
Baha Al-Shaikh, Sarah Hodge, Sanjay Agrawal, Michele Pennimpede, Sindy Lee, Janine MA Thomas, John Coombes
Combined pneumotachograph and Pitot tube design improves accuracy when measuring the inspired and expired tidal volume, compliance, airway pressures, volume/pressure and flow/volume loops. Modern devices can be used accurately even in neonates and infants.
Introduction
Published in Wilmer W Nichols, Michael F O'Rourke, Elazer R Edelman, Charalambos Vlachopoulos, McDonald's Blood Flow in Arteries, 2022
The French physicist/physiologist/physician EJ Marey made important contributions not only to noninvasive recordings of arterial pressure waves (Marey, 1863) but also to direct measurements of arterial blood flow. Marey’s textbook of 1881 includes a complete chapter on the velocity of blood flow in arteries. The technique used was that of a double Pitot tube, which is based on the Bernoulli principle and is used in modern aircraft to measure air speed. The Marey tracings look surprisingly like those that we accept as normal today, clearly showing the backflow phase of flow in the descending thoracic aorta, subclavian and femoral arteries (Hashimoto and Ito, 2013) (Chapter 9). As with Mahomed’s observations, these findings took another 100 years to be rediscovered and accepted. Marey’s special interest was in biological motion—of birds in flight and of running horses. His rapid-sequence photographs were the forerunner of modern cinematography (and coronary angiography). As with others previously mentioned, he was a master of many fields. The Wright brothers seasonal pilgrimages to Kitty Hawk beach in the Carolinas was to study the soaring of birds as observed by Marey in order to control their aircraft; with the engine and propeller the last additions before taking to the sky in powered flight (McCullough, 2015) back in Dayton, Ohio. Carl Ludwig appears to have been the first to introduce a continuously recording blood flow-meter. This was an ingenious though awkward device in which blood, flowing through an exteriorized tube, pushed before it a column of oil. As with his pressure recordings, this provided better registration of mean than of pulsatile flow. Adolph Fick is known for the principle he enunciated that related blood flow through an organ to the arteriovenous concentration gradient of a substance and the amount of the substance taken up or given off by that organ (Fick, 1870). This principle became the standard for measurement of cardiac output in humans following the first human cardiac catheterization by Forssmann (on himself) in 1929 and its popularization by Cournand, Richards and others in the early 1940s, and the developments which have followed, including the award of the Nobel Prize for physiology and medicine in 1956.
Development of a large-scale computer-controlled ozone inhalation exposure system for rodents
Published in Inhalation Toxicology, 2019
Gregory J. Smith, Leon Walsh, Mark Higuchi, Samir N. P. Kelada
We designed the system to have a nominal airflow rate of 250 L/min (15-air changes/hour/chamber) and a slight negative chamber pressure relative to the room to minimize the possibility of ozone entering the laboratory. We achieved these parameters by sequentially adjusting the supply and exhaust fan speeds and the system of control valves. While adjusting the system of fans and valves, we measured the static and differential pressures at specific locations of the air supply, chambers and exhaust. First, the operating pressure ranges of the chamber supply and exhaust manifolds were determined by running the supply blower and exhaust fans while the valves to the chambers were closed. After we achieved a sufficient difference between the supply and exhaust manifold static pressures (relative to the laboratory), we opened the chamber valves and adjusted them until the measurement of an orifice flow meter installed in the chamber exhaust corresponded to 250 L/min. The orifice flow meter we constructed consists of a flow restrictive orifice plate and a pressure transmitter connected to taps in the exhaust pipe to measure the differential pressure between up and downstream of the orifice. To generate a calibration curve for the orifice flow meter, we made airflow measurements with a pitot tube placed several feet downstream of the orifice and plotted these measurements against differential pressure measurements at a series of exhaust and supply valve settings. The calibration curve enables the maintenance of airflow at 250 L/min based on the corresponding orifice flow meter reading. To enable the monitoring of pressure readings directly (without the computer control software open), we mounted all of the static and differential pressure transmitters (6 units) in the door of a wall cabinet where they are visible. We also installed electrical conduit with wires to carry 4–20 mA current signals from the pressure transmitters to the opposite side of the laboratory. We terminated the wires at a USB microcontroller, which converts the 4–20 mA analog signal to a digital signal enabling visualization and recording on a computer.